Chairs provided with an adjustable back rest, adjustable alone or together with the seating portion if it forms a single body therewith, or synchronously with the seating portion but with a different angle of inclination are known. The purpose of this is to achieve the correct positioning for the particular work performed by the operator.
This adjustability of the inclination of the back rest, particularly alone, is generally obtained by hinging the rest support member about a transverse axis to the member mounted on the column of the chair base and supports the seating portion. The desired inclination of the back rest is achieved by locking the two members together in predetermined, angularly different positions.
However, in the case of inclination of the back rest together with or in synchronism with the seating portion, the adjustment is obtained by hinging together two separate parts forming the member for supporting the seating portion on the column. In the case of synchronized inclination between the seating portion and back rest, the back rest support member is hinged to one of the two parts forming the support member for the seating portion.
Considering the need to lock two hinged-together parts in different positions corresponding to different operator requirements, the system for locking these two parts together is generally a multiple type, i.e., with a locking bolt rigid with one of them and engaging a chosen one of several holes provided in the other part or alternatively with a comb structure rigid with one of them and engaging with a chosen one of the spaces between its teeth and an edge of the other part.
The most forwardly inclined position of the back rest is achieved by a spring between the back rest support member and the seating portion support member. The other rearwardly inclined positions are achieved by stress of the seated operator, and who by a rearward thrust, loads the spring until the desired inclination is achieved. This is permanently maintained by activating the locking system.
This system for elastically returning the back rest into its most forwardly inclined configuration involves the risk of sudden return into that configuration if the locking device is deactivated voluntarily or accidentally when the operator is not seated on the chair. This is the cause of possible danger and conflicts with current accident prevention regulations.
To eliminate this drawback it has already been proposed to make operation of the locking system (for activation or deactivation) possible only when the operator is seated on the chair and can oppose the elastic reaction on the return spring by his own presence.
For example, Italian utility model application BS91U000068 describes a chair adjustment device comprising a comb-type locking member activated and deactivated by a lever via a control rod provided with springs loaded by the operator seated on the chair, but which can move the comb member only after the operator has set the chair into the correct position for allowing this movement and has mechanically rigidly locked together the two parts of the support for the seating portion.
A drawback of this arrangement is that, if it is also required to adjust the height of the seating portion, a second lever must be provided, not only complicating the chair construction, but also causing operating difficulty and uncertainty because of the inevitable risk of confusing the two levers.
A further drawback, again related to the presence of the two levers, is that they have to be located on opposite sides of the chair so their operation requires the use of both hands.
A further drawback is that the lever for adjusting the inclination of the seating portion requires accurate construction and likewise accurate positioning on the chair. As the lever for adjusting the height of the seating portion is usually positioned on the right, the lever for adjusting the inclination of the seating portion is constructed to be always positioned on the left and could not be positioned on the right even if the lever for adjusting the height of the seating portion is not provided.
A further drawback is that the maneuvers to be carried out on the lever to lock and release the seating portion are opposing, i.e., are different from each other. This results in a possible operational uncertainty by the operator, who cannot always immediately remember which of the two operations has to be carried out to lock or release the seating portion.
A further drawback is that the system for operating the manual locking member of a rigid rod involves the need to apply the control lever in a precise position, which is generally in proximity to the inclination mechanism of the seating portion.
A further drawback is a certain complexity of construction and assembly, both of the locking member and of its operating device.
According to the invention, all these drawbacks are eliminated through a device for adjusting the inclination of the seating portion in chairs.